Transection of the canine anterior cruciate ligament (ACLT) is a well- established means of inducing osteoarthritis (OA), which develops slowly and is mild for the first 2-3 years after surgery. However, the progression of OA can be greatly accelerated if hind limb deafferentation, accomplished by ipsilateral dorsal root ganglionectomy (DRG), precedes ACLT. Since dogs with stable joints do not develop joint pathology after DRG, it seems likely that the nervous system uses 2 separate mechanisms to protect joints: 1) stable joints are protected by centrally programmed mechanisms that do not depend upon proprioceptive or nociceptor information from ipsilateral sensory nerves; and 2) unstable and/or injured joints are protected from rapid breakdown by sensory information carried by ipsilateral nerves. In both cases, it is likely that co-ordinated muscular activity (protective muscular reflexes, or PMRs) is responsible for maintaining the range of excursion within the safest possible limits, thereby protecting the joint. The purpose of this study is to determine if alterations in kinematics and electromyography (EMG) can be used to investigate the mechanisms by which sensory nerves control neuromuscular reflexes that protect joints. Four groups of 7 dogs will be used: Group A will undergo unilateral ACLT to destabilize the knee joint; Group B will undergo sham ACLT; Group C will undergo unilateral DRG to deafferent the hind limb; Group D will undergo sham DRG. All animals will be trained to locomote on a motor-driven treadmill. Kinematic and EMG evaluations will be performed preoperatively, and at 4, 8, 12, and 24 weeks postoperatively. The kinematic evaluation will use high-speed films to examine changes in joint angular excursion and velocity patterns as well as stance/swing relationships, whereas the EMG analysis will examine changes in muscle amplitude and on-off patterns. These changes will be regarded as the "normal" response of the neuromuscular system to ACLT and DRG, and will provide baseline information critical for future experiments designed to determine how the nervous system organizes PMRs and protects joints.